Field of the Invention
The present invention relates to a post regulation control circuit for a switch mode power supply (SMPS) with multiple outputs, said circuit being of the phase modulation trailing edge synchronisation type having a ramp generator and a triggering mechanism.
Generally such switch mode power supplies take electrical power from a source as an input voltage and deliver electrical power to multiple loads.
The invention is particularly directed to isolated or non-isolated single-ended or double-ended converters.
Typically they can be buck-derived isolated or non-isolated SMPS which take power from a source at a voltage Vin, and deliver power to a load, at a voltage Vo. Typically the output voltage is regulated in order to guarantee that a constant voltage is applied to the load even if the input voltage Vin or the output current Io or both vary. Often it is desirable to add one or more outputs Vo2, Vo3, and so on. Again these individual output voltages should be independently regulated.
Many proposals have been made to address this problem. For example, if the load current of the auxiliary output is relatively low a linear regulator can be used. While this approach is simple and gives excellent results in terms of output noise and dynamic performance it suffers from poor efficiency particularly if wide input voltage variations have to be accommodated.
Magnetic amplifier output regulators have been used as a means for regulating more than one output of a switching supply. They are particularly suitable for currents of 1 amp to more than 20 or 30 amps, though they have been used where tight regulation and efficiency are extremely important at lower current ratings. They are efficient in that they provide extremely precise regulation of each independent output and are simple and reliable. Closely regulating outputs can be obtained using a simple control circuit. However, the magnetic amplifiers tend to be bulky and expensive. Moreover they can be lossy particularly if the switching frequency is high. Minimum delay times do cause duty cycle losses, and reduce the maximum achievable output voltage for the auxiliary outputs. A typical magnetic amplifier and the design of such high frequency magnetic amplifier output regulators is described in an application guide produced by Allied Signal Inc. and entitled `Design of High Frequency Mag Amp Regulators Using Metglas` (see 1977 Allied Signal Inc. R697/1.5M)
Magnetic amplifier regulators have problems in that because they have to withstand the maximum input voltages during a short circuit condition, they are effectively over-designed, typically by a factor of two which increases the cost and size of the power supply. Secondly, they are inherently leading edge modulators so that you can only approach a certain maximum duty cycle limited by the minimum delay and the magnetic BH loop characteristics of the magnetic amplifier core. This forces an increase in the size of the main transformer as well as the output inductor resulting in high overall system costs.
Alternatively, one or more buck converters can be connected to the main output Vo1 and deliver independently regulated outputs. Good results in terms of efficiency can be achieved at the expense of increased output noise at the main output Vo1. Additionally the pulsating input current into the buck stage causes a high ripple current stress of the output filter capacitor of the main output. The buck converter can be replaced by a current-fed converter with a continuous input current. The output noise problems can be solved, but new problems are introduced, for example, poor boost switch utilisation.
The noise problems at the main output mentioned above can be avoided if the auxiliary outputs are not directly derived from the main output but from the pulsating voltage at the secondary of the transformer. Regulation of the auxiliary output is achieved by phase modulation in combination with a controlled rectifier.
Various solutions to obviate the rise of a magnetic amplifier have been proposed. It is known to provide a phase modulated synchronous secondary side regulator which overcomes the propagation delay time. Such a controller is produced by Linfinity Microelectronics and is described in their data sheet LX1570/1571 Copyright .COPYRGT.1995 Linfinity Microelectronics Inc. A controlled forward rectifier is turned on before the start of the power cycle. Depending on the error of the output voltage of the auxiliary output, the controller forward rectifier is turned off sometimes during the power cycle. Leading-edge phase modulation between the main output and the auxiliary output is achieved. The turn-off of the controlled forward rectifier can coincide with the end of the main power cycle, and duty cycle losses can be completely avoided. A significant problem, however, is introduced to the converter. Leading-edge pulse with modulation means that at the beginning of the power cycle the primary current equals the reflected sum of the output currents. After the controlled forward rectifier is turned off the primary current drops to a value equal to the reflected main output current. This shape of the primary current is no longer compatible with primary-peak-current-mode-control (PPCMC) as the most popular SMPS control scheme. Voltage mode control or average current mode control has to be used, each with its own set of problems.
In another phase modulation approach to the problem, the post regulator control circuit ensures that the trailing edges of the power pulses into the main and auxiliary outputs are synchronised. A typical example of such an approach is a secondary side post regulator for AC/DC and DC/DC multiple output converters manufactured by Cherry Semiconductor Corporation and identified under the product No. CS5101 and described in a data sheet of the same title, copyright .COPYRGT.1995 Cherry Semiconductor Corporation. The post regulator control circuit ensures that the trailing edges of main and auxiliary outputs are synchronised. A ramp is generated, and triggered at the start of each power cycle. Depending on the error of the output voltage of the auxiliary output, a delay between the start of the main power cycle and the turn-on of the synchronous switch is generated. Trailing-edge phase modulation between the main output and the auxiliary output is achieved. While good efficiency figures, good regulation and low output noise can be achieved the scheme shares one disadvantage with magnetic amplifiers. Inherently there will be control circuit propagation delays between the detection of the start of the power cycle to turn-on of the controlled forward rectifier. This results in a duty cycle loss of the auxiliary output with respect to the main output. Since these propagation delays are inherent to the control circuit, and constant versus the switching frequency of the SMPS, the problem gets worse at higher switching frequencies.
Typical multiple output converters are described in U.S. Pat. Nos. 5,005,121 (Mitsubishi), 5,541,828 (AT&T) and 5,862,042 (Lucent Technologies).
There is an increasing demand for converters to supply multiple loads at different level of voltages over a large range. Typically loads do not tolerate wide variations in their supply voltages and therefore the output voltages of these converters has to be well regulated.
In this specification reference is had to trailing edge synchronisation. However, when talking about modulation people skilled in the art may refer to this as leading edge modulation. Thus, in FIG. 3 the trailing edges of Vrect1 and Vrect2 are synchronised and similarly it is correct to say their leading edges are modulated.
The present invention is directed towards providing an improved construction of a post regulation control circuit for a switch mode power supply with multiple outputs of a phase modulation type having trailing edge synchronisation, a ramp generator and a triggering mechanism.